Several methods of liquefying a natural gas stream thereby obtaining liquefied natural gas (LNG) are known.
It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form, because it occupies a smaller volume and does not need to be stored at high pressures.
Usually, the natural gas stream to be liquefied (mainly comprising methane) contains ethane, heavier hydrocarbons and possibly other components that are to be removed to a certain extent before the natural gas is liquefied. To this end, the natural gas stream is treated. One of the treatments involves the removal of at least some of the ethane, propane and higher hydrocarbons such as butane and propanes (often referred to with “NGL extraction” or “NGL recovery”).
U.S. Pat. No. 5,291,736 discloses a known method of liquefaction of natural gas including the removal of hydrocarbons heavier than methane. Another example of a known method is given in US 2005/0247078.
A problem of the known methods is that if a relatively lean feed stream (i.e. containing relatively little ethane, propane and other hydrocarbons) is to be processed, no optimal use is made of the available cooling capacity. In other words, less LNG is produced using the same cooling duty.